1. Field of the Invention
This invention relates to an energy subtraction processing technique, wherein a plurality of radiation images having been formed respectively with a plurality of kinds of radiation having different energy distributions are obtained, and wherein a specific image constituent embedded in the radiation images is separated by use of the obtained radiation images.
2. Description of the Related Art
Techniques for performing energy subtraction processing have heretofore been known in the field of medical image processing. With the energy subtraction processing techniques, a plurality of radiation images carrying image information of a single object are formed with a plurality of kinds of radiation having different energy distributions. Thereafter, image signal components of image signals representing the plurality of the radiation images, which image signal components represent corresponding pixels in the radiation images, are multiplied by appropriate weight factors, and the thus weighted image signal components are subjected to a subtraction process. From the subtraction process, a difference signal, which represents only the image of a specific structure of the object, is obtained. By the utilization of the energy subtraction processing techniques, it is possible to form a soft tissue image, in which a bone constituent has been removed from the input images, and a bone image, in which a soft tissue constituent has been removed from the input images. Therefore, image reading may be performed on the image, in which the area other than the area to be diagnosed has been removed, and the perceptibility of the area to be diagnosed in the image is enhanced. (Reference may be made to, for example, U.S. Pat. No. 6,624,439.)
FIG. 12 is an explanatory view showing a scattered radiation removing effect of a grid. As illustrated in FIG. 12, at the time at which the radiation having been irradiated from a radiation source passes through an object, the radiation attenuates due to two effects, i.e. a photoelectric effect and a Compton effect. Part of the radiation having attenuated due to the Compton effect undergoes a change in direction of travel and constitutes scattered ray components. The scattered ray components cause the level of contrast of the radiation image to become low. Therefore, a grid is located between the object and a detector for detecting the radiation image information of the object, and the scattered ray components are thereby suppressed at the time of an imaging operation.
The energy subtraction processing apparatus disclosed in U.S. Pat. No. 6,624,439 is provided with an imaging system in which a grid is located. With the disclosed energy subtraction processing apparatus, energy subtraction processing is performed by use of radiation images having been formed with the radiation, from which the scattered ray components have been removed by the grid.
As radiation imaging apparatuses utilizing flat panel detectors have become popular in recent years, it is becoming possible to perform the imaging operations for energy subtraction processing with respect to various object sites to be imaged. At the time of the imaging operations described above, a tube voltage and grid use conditions, such as the presence or absence of the grid and the kind of the grid, are adjusted in accordance with the object sites to be imaged.
However, in the cases of the conventional energy subtraction processing as described in, for example, U.S. Pat. No. 6,624,439, wherein the object site to be processed is limited, the grid use conditions at the time of the imaging operations are set fixedly, and therefore it has not been necessary for an effect of a difference in grid use conditions upon the energy subtraction processing to be taken into consideration in the energy subtraction processing.